IXIOLITE
IIxiolite – (Ta,Nb,Sn,Fe,Mn)4O8 , referring to members of the Ixiolite-(Mn²⁺) – Ixiolite-(Fe²⁺) Series – is a rare oxide found exclusively in granitic pegmatites, where it occurs in association with columbite and tantalite. It is a tantalum ore containing up to 69% Ta₂O₅ (in its pure form) and is a common mineral in coltan ores.
It contains traces of Zr, Hf, Ti, and W. There is a variety rich in Sc (up to 4%), which is typically also rich in Ti and Sn (up to 16% SnO2). Another variety contains W, and a third has a high Nb content; the latter is likely a new mineral species. The designation “pseudo-ixiolite” is currently considered a synonym for ixiolite.
Its high density – comparable to that of tantalites – is a diagnostic feature. Crystal morphology strongly resembles that of columbites, with which it is easily confused. It can form crystals with a shape reminiscent of the tourmaline habit.
Information on ixiolite in ore mineral literature is very fragmentary; many classic texts do not even cover it. Other databases also simply ignore the mineral, despite it being a common constituent of a strategic ore: COLTAN.
1. Characteristics
Crystal system: Orthorhombic
Color: Steel gray to black.
Habit: Idiomorphic crystals up to 10 cm. Spherical aggregates, rectangular prisms formed by pinacoids.
Cleavage: No.
Tenacity: Brittle.
Twinning: On {103}, sometimes.
Fracture: Irregular, subconchoidal.
Mohs Hardness: 6 – 6.5
Parting: No information available.
Streak: Dark brown.
Lustre: Submetallic.
Diaphaneity: Opaque.
Density (g/cm³): 7.03 – 7.23
2. Geology and Deposits
Ixiolite is characteristic as an accessory mineral in Ta-rich granitic pegmatites.
3. Mineral Associations
At the type locality (Skogsböle, Finland), it occurs with tapiolite-(Fe) and feldspars.
It associates with minerals common in granitic pegmatites, such as quartz, albite (including the cleavelandite variety), microcline, beryl, muscovite, spodumene, and cassiterite.
It occurs with minerals typical of the specific paragenesis, such as tantalite, microlite, monazite-(Ce), cassiterite, and Nb-rutile.
It also occurs with radioactive minerals (thorite) and rare minerals (wodginite, titanowodginite).
4. Transmitted Light Microscopy
Not applicable, as ixiolite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: There is no information regarding the hardness and polishing behavior of ixiolite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Gray.
Pleochroism: Ixiolite is generally metamict, and it is no longer possible to observe this property.
Reflectivity: 13-14% (Very low, similar to common silicates!)
Bireflectance: Ixiolite is generally metamict, and it is no longer possible to observe this property.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Ixiolite is generally metamict, and it is no longer possible to observe this property.
Internal reflections: Yellow to dark red.
May be confused with: No information available.
General Characteristics:
Grain shape: forms elongated tabular crystals. It can also form isometric and circular grains.
Cleavage in one direction is visible.
Polysynthetic twinning occurs.
Inclusions of ixiolite in other minerals may occur.
Vermiform intergrowths between ixiolite-(Ti) and Nb-rutile may occur. “Flame-like” intergrowths between ixiolite and tantalite-(Fe) may also occur.
